A sensor for monitoring materials is taught in the applicant's U.S. Pat. No. 5,795,064, issued Aug. 18, 1998 to Mathis, the entire disclosure of which is hereby incorporated by reference. This sensor provides a non-destructive test due to the surface measurement and interfacial nature of the sensor interaction with the sample. The sensor measures a materials effusivity (the square root of thermal conductivity, density and heat capacity).
The sensor of the above identified patent can also be used for direct measurement of thermal conductivity as taught is the applicant's U.S. Pat. No. 6,676,287, the entire disclosure of which is hereby incorporated by reference.
Referring to FIG. 1 there is illustrated a known material monitoring sensor. The sensor 10 includes a hot wire 12 and guard heaters 14.
Referring to FIG. 2 there is illustrated heat flow from the sensor of FIG. 1.
In operation, a known quantity of electrical current is passed through the heating elements 12 and 14 of the sensor for a known time. This results in a temperature rise at the sensor/sample interface and, over time, a heat flow from the sensor into the sample. The sensor functions by measuring the temperature rise at the sensor/sample interface over time. The heat transfer properties of the sample profoundly affect the rate of this temperature rise. If the sample is a good thermal insulator, then as heating continues, very little heat is conducted away from the sensor/sample interface and the temperature at the interface rises very quickly. If the sample is a good heat conductor, then as the heating continues, the heat is conducted away from the sensor/sample interface and the temperature at the interface rises very slowly.
The heating elements and control mechanisms are designed to keep the sensor/sample interface temperature rise within certain boundaries. Temperature rise can also be controlled by adjusting the test time. A calibration curve is constructed by performing tests on standard materials with known thermal effusivity and/or thermal conductivity. Once the calibration curve is determined, samples are tested under identical experimental conditions, and the rate of temperature change at the sensor/sample interface is translated directly into thermal effusivity and/or thermal conductivity.
The sensor for FIG. 1 uses tightly controlled heating at the surface of a sample to make direct measurements of thermal effusivity and/or thermal conductivity. The apparatus applies a known quantity of heat for a known time to the surface of a sample. During testing, three basic things happen to the applied heat: some of the heat goes into the backing material, most of the heat goes into the sample, and some of the heat goes nowhere and causes a localized temperature increase at the sensor/sample interface. The magnitude of the temperature rise at the sensor/sample interface can be quantitatively converted to thermal effusivity and/or thermal conductivity because the rise is completely dependent on the heat transfer properties of the material. As shown in FIG. 2, the heat flow from the hot wire 12 into a material being tested is initially straight as indicated by an arrow 16, due to the heat flow from the guard heaters 14 as indicated by curved arrows 18. However, the heat flow then diverges as indicated by arrows 20.
Consequently, the sensor is highly suitable for static measurements as the sensor must remain in stable contact with the material being measured. Unfortunately, many processes in which material measurements are desirable are dynamic in nature.
While providing a valuable tool for sample measurement, the sensor's structure and geometry result in a measurement period in the order of two to ten seconds. For processes such as mixing, using for example a V blender, the blender must be stopped in a particular orientation to allow the material being mixed to settle and to contact the sensor in order to take a reading. This results in a time delay for each measurement, which cumulatively adds to the total mixing time and actually disturbs the mixing process.
Consequently, there is a need in the prior art for a method and apparatus for monitoring materials during dynamic processes.